1. Field of the Invention
The present invention relates to an automatic transmission using a planetary gear mechanism.
2. Description of the Related Art
Conventionally, there is known an automatic transmission capable of providing shift transmission with eight forward speed stages and one backward speed stage using four planetary gear mechanisms and six engagement mechanisms including clutches and brakes (Refer to, for example, Japanese Patent Application Laid-Open No. 2012-97864, hereinafter Patent Document 1).
The automatic transmission in Patent Document 1 includes an input shaft rotatably supported in a housing and an output member including an output gear arranged coaxially with the input shaft. The rotation of the output member is transmitted to the right and left driving wheels of a vehicle through a differential gear or a propeller shaft.
Four first to fourth planetary gear mechanisms are arranged coaxially with the input shaft in the housing. The first planetary gear mechanism is a so-called single-pinion type planetary gear mechanism that includes three elements: a first sun gear, a first ring gear, and a first carrier supporting a first pinion meshing with the first sun gear and the first ring gear in such a way that it is rotatable about its axis and about the first sun gear (if the sun gear is rotated with the carrier fixed, the ring gear rotates in a different direction from the direction of the sun gear, and thus the planetary gear mechanism is also referred to as “minus planetary gear mechanism” or “negative planetary gear mechanism”; if the sun gear is rotated with the ring gear fixed, the carrier rotates in the same direction as the sun gear).
Assuming that three elements of the first planetary gear mechanism are defined as a first element, a second element, and a third element in sequence of a collinear diagram (a diagram which illustrates a ratio of relative rotation speeds of the three elements of the sun gear, the carrier, and the ring gear using straight lines [speed lines]) for the first planetary gear mechanism from one side, the first element is the first sun gear, the second element is the first carrier, and the third element is the first ring gear.
The second planetary gear mechanism is also a so-called single-pinion type planetary gear mechanism that includes a second sun gear, a second ring gear, and a second carrier supporting a second pinion meshing with the second sun gear and the second ring gear in such a way that it is rotatable about its axis and about the second sun gear. Assuming that three elements of the second planetary gear mechanism are defined as a fourth element, a fifth element, and a sixth element in sequence of a collinear diagram for the second planetary gear mechanism from one side, the fourth element is the second ring gear, the fifth element is the second carrier, and the sixth element is the second sun gear.
The third planetary gear mechanism is also a so-called single-pinion type planetary gear mechanism that includes a third sun gear, a third ring gear, and a third carrier supporting a third pinion meshing with the third sun gear and third ring gear in such a way that it is rotatable about its axis and about the third sun gear. Assuming that three elements of the third planetary gear mechanism are defined as a seventh element, an eighth element, and a ninth element in sequence of a collinear diagram for the third planetary gear mechanism from one side, the seventh element is the third sun gear, the eighth element is the third carrier, and the ninth element is the third ring gear.
The fourth planetary gear mechanism is also a single-pinion type planetary gear mechanism that includes a fourth sun gear, a fourth ring gear, and a fourth carrier supporting a fourth pinion meshing with the fourth sun gear and fourth ring gear in such a way that it is rotatable about its axis and about the fourth sun gear. Assuming that three elements of the fourth planetary gear mechanism are defined as a tenth element, a 11th element, and a 12th element in sequence of a collinear diagram for the fourth planetary gear mechanism from one side, the tenth element is the fourth ring gear, the 11th element is the fourth carrier, and the 12th element is the fourth sun gear.
The first sun gear (first element) in the first planetary gear mechanism is connected to the input shaft. The fourth ring gear (tenth element) in the fourth planetary gear mechanism is connected to the output member.
The first carrier (second element) in the first planetary gear mechanism, the second carrier (fifth element) in the second planetary gear mechanism, and the third ring gear (ninth element) in the third planetary gear mechanism are connected to each other, by which a first connected body (second element, fifth element, and ninth element) is formed. Further, the first ring gear (third element) in the first planetary gear mechanism and the fourth sun gear (12th element) in the fourth planetary gear mechanism are connected to each other, by which a second connected body (third element and 12th element) is formed. Furthermore, the third carrier (eighth element) in the third planetary gear mechanism and the fourth carrier (11th element) in the fourth planetary gear mechanism are connected to each other, by which a third connected body (eighth element and 11th element) is formed.
Moreover, the automatic transmission in Patent Document 1 has six engagement mechanisms, in total, including three first to third clutches and three first to third brakes.
The first clutch is a wet multi-plate clutch and is switchable between a connected state where the first sun gear (first element) in the first planetary gear mechanism is connected to the third connected body (eighth element and 11th element) and a release state where the connection is released. The second clutch is a wet multi-plate clutch and is switchable between a connected state where the first sun gear (first element) in the first planetary gear mechanism is connected to the second ring gear (fourth element) in the second planetary gear mechanism and a release state where the connection is released.
The third clutch is a wet multi-plate clutch and is switchable between a connected state where second sun gear (sixth element) in the second planetary gear mechanism is connected to the second connected body (third element and 12th element) and a release state where the connection is released. A first brake is a wet multi-plate brake and is switchable between a fixed state where the third connected body (eighth element and 11th element) is fixed to the housing and a release state where the fixing is released.
A second brake is a wet multi-plate brake and is switchable between a fixed state where the third sun gear (seventh element) in the third planetary gear mechanism is fixed to the housing and a release state where the fixing is released. The third brake is a wet multi-plate brake and is switchable between a fixed state where the second sun gear (sixth element) is fixed to the housing and a release state where the fixing is released.
In the automatic transmission in Patent Document 1, a first forward speed stage is established by placing the first, second, and third brakes in the fixed state. A second forward speed stage is established by placing the second and third brakes in the fixed state and placing the third clutch in the connected state. A third forward speed stage is established by placing the second and third brakes in the fixed state and the second clutch in the connected state. A fourth forward speed stage is established by placing the second brake in the fixed state and the second and third clutches in the connected state.
A fifth forward speed stage is established by placing the second brake in the fixed state and the first and second clutches in the connected state. A sixth forward speed stage is established by placing the three first to third clutches in the connected state. A seventh forward speed stage is established by placing the third brake in the fixed state and the first and second clutches in the connected state. An eighth forward speed stage is established by placing the third brake in the fixed state and the first and third clutches in the connected state. A backward stage is established by placing the first and third brakes in the fixed state and the second clutch in the connected state.